Conventionally, cast metal restorations are made by a process involving embedding a wax pattern in an investment material, eliminating the wax pattern from the investment mold, and thereafter producing a metal casting from the mold. Typically, a wax or plastic sprue is attached to the wax pattern before it is invested, so that during the wax elimination step a channel known as a sprue channel, provides an escape route for molten wax and its residue during the wax elimination. More specifically, in a conventional procedure, the sprue channel provides an escape route from the investment mold that terminates near the centerpoint of the bottom surface of the investment mold.
Conventionally, the wax pattern is eliminated by burnout of the mold, and a burnout furnace having an electric muffle (or heating chamber) is used. To create an oxidizing atmosphere in the burnout furnace, an open vent hole is usually located high in the back wall of the furnace. When the wax pattern melts and burns, it leaves a residue of carbon, and at elevated temperatures, oxygen forms carbon monoxide and carbon dioxide from the carbon residue.
Thereafter, in a conventional procedure, a metal casting is made from the mold by use of a casting machine. A commonly used casting machine is a centrifugal casting machine in which the rotation is in a horizontal plane. Porcelain may be used to cover the metal casting, and ultimately a dental restoration is put into a patient's mouth. Full cast crowns are waxed to exact dimensions and are finished and polished after casting.
Different types of investment materials are known, and include thermal investments. Typically, for a thermal investment, the wax pattern is invested in a casting ring and allowed to set for a minimum of about 60 minutes before being placed in the burnout furnace. Usually, the furnace is initially at room temperature and is heated slowly during a period of about 4.5 hours to an elevated temperature of 1200.degree. or 1600.degree. F. To eliminate the wax pattern, this elevated temperature is generally maintained for a period of 1 hour minimum, and the total burnout time is about 270 to 280 minutes, plus about 15 minutes extra for each additional ring.
Although this conventional process is widely used and has great acceptance, an improved process for making cast metal and porcelain fused to alloy restorations is needed. For example, the conventional process, and in particular the technique for burnout of the mold, is time intensive. Benefits would be obtained by reducing the time in terms of both labor and energy savings.